Dinosaurs, according to the prevailing scientific theory, met their dramatic end after an enormous asteroid hit the Earth near what is now Mexico. It's easy to imagine that this mass extinction, known today as the Cretaceous-Tertiary Mass Extinction event, or K-T event, left nothing recognizable alive on Earth. The impact itself was catastrophic, though it probably had some help from volcanoes and other factors in wiping out 70 percent of all life on the planet [source: NASA Jet Propulsion Laboratory].

The dinosaurs were, of course, the most famous of the life forms that died out post-asteroid. But other reptilian animals became extinct as well. These included aquatic reptiles like plesiosaurs, ichthyosaurs and mosasaurs. The first vertebrate animals ever to learn to fly by flapping their wings -- the pterosaurs -- vanished after the K-T event, too. So did 90 percent of algae species and vast numbers of oceanic invertebrates [source: University of Bristol]. Life was, quite literally, not the same.

­But some types of animals weren't hit nearly as hard. At least a few members of about 84 percent of marine families and 82 percent of land vertebrate families made it through [source: Space.com]. In fact, so many life forms survived the K-T event that it would take a textbook to describe them all. Many of these animals have descendants that live tod­ay.

Some of today's species look a lot like their Mesozoic counterparts. Others are the few surviving members of scientific families that lived millions of years ago. And then there's the theory about dinosaurs that survived the K-T event by evolving into birds, which you can read more about in How Dinosaurs Work.

So, aside from birds, which of today's animals walked, crawled, slithered or swam alongside dinosaurs? Next, we'll look at the land animals that shared territory with the terrible lizards and their contemporaries.

Land Animals that Walked with Dinosaurs

A zookeeper cradles rare twin platypus babies. The platypus is one of the few remaining monotremes on Earth.

Dinosaurs lived on Earth during the Mesozoic Era, which lasted from 248 to 65 million years ago. Geologists divide the Mesozoic Era into three periods: from longest ago to most recent, the Triassic, Jurassic and Cretaceous. Dinosaurs became more diverse as time went by, and at the same time, other life forms both developed and became extinct.

If the Mesozoic Era was the age of dinosaurs, the next era -- the Cenozoic Era -- was the age of mammals. In fact, a past theory about the extinction of the dinosaurs was that newly evolving mammals ate all their eggs. This is partly because most early mammals were small, so dinosaur eggs may have made a tasty and convenient meal for them. The first mammals were monotremes, or mammals that reproduce by laying eggs. Mammals are common today, but only three monotreme species still exist. These are the duck-billed platypus and a couple of spiny anteaters, or echidnas.

Many reptile species died during the K-T event, but snakes, lizards and the crocodilians persevered. Crocodilians have been on the planet for about 240 million years. There are 23 crocodilian species today, including alligators, crocodiles and caimans. Mesozoic crocodilians were generally larger than living species. Another prehistoric order of reptiles is the sphenodontians. Today, there is one living sphenodontian -- the tuatara, which lives in New Zealand.

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The tuatara (Sphenodon punctatus) is the only surviving species of an order that flourished 200 million years ago in New Zealand.

The most likely evolutionary origin for these reptiles is that in the distant past they arose from amphibians. Very large amphibian species lived before and during the Mesozoic. These are gone today, but in their place three primary types of amphibians survived:

Unlike monotremes and sphenodontians, not every family that lived during the Mesozoic has only a few descendents left today. All modern insect groups either existed before or arose during the Mesozoic Era. Perhaps most notable of these is the eusocial bee -- bees that live in colonies. Most likely, these evolved along with flowering plants, or angiosperms, which started to develop in the Cretaceous Period. Without this co-development, neither humans nor bears would have easy access to honey today.

The oceans were particularly devastated by the K-T event. We'll look at the oceanic survivors that are part of Earth's modern seas on the next page.

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Mesozoic Plants

Angiosperms weren't the only important plants during the Mesozoic Era. Ferns and conifers, for example, were widespread, and ferns experienced a huge population spike after the K-T event. Then there's the Gingko biloba. If you see one of these trees, which has fan-shaped green leaves that turn golden yellow in the fall, you're looking at a plant that is almost identical in appearance to its Mesozoic ancestors.

Marine Animals that Swam with Dinosaurs

Except for possible excursions to find food, reproduce or just cool off, dinosaurs were not aquatic. However, there were lots of sea-dwelling animals during the Mesozoic Era. Plesiosaurs were long-necked, finned reptiles. Think of the Loch Ness Monster, and you've got a pretty good idea of what one looked like. Ichthyosaurs looked more like dolphins, and mosasaurs bore a passing resemblance to thick-bodied, finned eels with overlapping scales.

Along with many other marine life forms, all of these animals became extinct during the K-T event. Many members of some of today's marine groups also experienced heavy losses. Some of these included:

After the K-T event, these and other marine life forms eventually recovered and went on to thrive and diversify. The clams, snails, lobsters, crabs and shrimp that make their way to the mouths of oceanic carnivores and the plates of hungry humans come from predecessors that, one way or another, lived through the K-T event. Sharks also inhabited the world's oceans long before the first dinosaur made its way across the land, and they're among the most well-known oceanic predators today.

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Although they didn't exist during the Mesozoic era, lampreys bear a resemblance to Mesozoic sea life.

But a few of today's species are the only remaining examples of long-extinct marine families. Today's hagfish and lampreys bear a resemblance to the now-extinct ostracoderms, which were jawless fish. But the most well-known throwback to Mesozoic marine life may be the coelacanth, the last known marine sarcopterygian. Sarcopterygians were lobe-finned, bony fish. There are plenty of other sarcopterygians in the world, though -- the four-legged, vertebrate tetrapods arose from ancestors that diverged from the sarcopterygians long before the Mesozoic Era.

Scientists thought coelacanths were extinct until the 1930s, so it's possible that researchers may one day find other remnants of Mesozoic life. To learn more about the possibilities, see the links on the next page.